Ab initio spin-orbit configuration interaction (CI) calculations on uranium, which is modeled with relativistic effective core potentials (ECP) and Gaussian cc-pVDZ and cc-pVTZ basis sets, are in progress for both free and doped ions. The best calculations, MRCISD, on the tetravalent uranium free ion, which has two 5f electrons, have a maximum relative error of 9.6% when compared to the experimental spectrum. In an effort to identify the sources of this error as well as to elucidate an anomalous uranyl ion bond length reported by the PNNL group using the Ermler U ECP, a study of uranium ECPs was initiated.

Phillip Christiansen developed two new U ECPs: a 78 electron ECP, where the valence space is 6s 6p 5f 6d 7s containing 14 electrons for the neutral, and a 68 electron ECP, where the valence space is 5d 6s 6p 5f 6d 7s containing 24 electrons for the neutral. This development incorporates improved fitting of the spin-orbit operators wherein the Gaussian exponents for the spin-orbit expansion may differ from those exponents used to expand the core potential. The Ermler U ECP has a 78 electron core. The table below shows the energy levels for the tetravalent uranium free ion at the cc-pVDZ MRCISD level of theory. For a particular energy level the ECPs can give significantly different values. Other calculations have shown that the majority of the difference between the ECPs, regarding the energy levels, is from the spin-orbit operators.

SCF and CIS calculations on the uranyl ion with a cc-pVDZ O basis set yield equal bond lengths to one thousandths of a bohr provided the U basis set is VTZ or better. (8sd 7p 8f 2g 1h)/[8sd 7p 8f 2g 1h] for U is close to the basis set limit and gives bond lengths of 3.084, 3.091, and 3.127 bohr for the Ermler, the 78 Christiansen, and the 68 Christiansen ECPs, respectively. Bert De Jong's DF bond length is 3.120 for U TZ and O DZ. This suggests that for accurate calculations a U ECP with the 5d shell in the valence space is required. Calculations with the 5p and/or the 5s 5p in the valence space are planned.